The present invention relates to analog-to-digital converters and, more particularly, to integrating analog-to-digital converters.
Analog signals generated by various devices such as sensors are very often desired to be converted into corresponding digital signals because of the convenience and accuracy of digital signal processing. Analog-to-digital converters are used to provide sequences of digital code representations of numbers in a chosen number base such that the numbers correspond to the amplitude value of points in, or parts of, an analog signal input waveform provided with respect to some reference value. In other words, the digital code representations of a sequence of numbers is related to the magnitudes of a corresponding sequence of selected points in, or to the magnitudes of the averages of some sort found for each of a corresponding sequence of selected parts of, such an analog input signal with respect to the magnitude of a reference level.
Such analog-to-digital converters compare such magnitudes occurring in an analog input signal at selected times to a conversion reference level magnitude in an attempt to provide an approximation of this relationship, as it occurs in a short sampling time interval or over an averaging duration, by a digital code representation. This process is usually repeated periodically to give a sequence of digital code representations corresponding to sample points or parts in this analog input waveform. The conversion process may be expressed by the analog input signal magnitude, or ratio of signal and reference magnitudes, being taken equal to the product of the conversion reference level magnitude, the output "estimating number" that is represented by the digital code representation, and a "transfer function parameter" which is just equal to one for direct linear converters. However, several possibilities in a converter can permit variances to occur which result in a conversion process leading to a non-linear converter if the design of the converter is not carefully managed. In addition, the transfer function may be intended to perform a mathematical process such as integration so that the output digital code representations represent the integral of the analog input signal.
Another source of difficulty in the conversion process is the presence of noise superimposed on the analog input signal to be converted to a sequence of digital code representations. Since a direct conversion process, as previously described, provides a digital code representation for each corresponding selected sample point in the analog input signal, and so depends on the value of that input signal at the exact point in time when the sample is taken, the output code sequence will usually differ from what it otherwise would have been in the absence of noise superimposed on the analog signal. Although such noise could be removed to a considerable extent by preliminary analog filtering or subsequent digital processing, there can be substantial value in eliminating any effect of the noise before the conversion is complete. Typically, this is done by using an analog-to-digital conversion technique in which the digital code presentation depends on the time integral, or average value, of the analog input signal during some time interval about each point where a conversion is desired. Such integration, or averaging, of the signal sample leads to being able to give very repeatable results for the same analog waveform even in the presence of substantial amounts of noise occurring in connection with that signal. The effects of noise will be averaged out for those noise frequencies present within the analog input signal which have the reciprocal values thereof that are less than the time of integration of the analog input signal about a sampling point.
The time interval of such integration, if done discretely for each sample part, must be small enough to insure that there will be a sequence of digital code representations at a rate sufficient to successfully simulate the analog input signal without any aliasing thereof. On the other hand, the time interval should be as large as possible under this constraint to the extent possible so as to maximize the duration of signal averaging for each point which will in turn minimize the noise induced errors in the output digital code representations.
In some situations, there is the need to have an average value of the input analog signal, averaged over many sampling times thereof, be very accurately provided at the output of the converter, and that accuracy must be maintained as precisely as the accuracy of the average value determined over one or a few sample periods. That is, the errors in each sample must not accumulate over many such samples. One such situation is the use of rate gyroscopes in aircraft to provide attitude and heading reference signals. In such systems, the precision of the angular rate measurement must be on the order of one or two tenths parts per million, which requires the analog-to-digital converter to be able to provide output signals of 24-bit precision if that accuracy is to be maintained. This requirement can be eased somewhat if a selectable gain amplifier is used ahead of the analog-to-digital converter to effectively compress the range of the analog input signal. However, the gain changing range for such an amplifier is often limited to a 10:1 range, and this results in still requiring a 22-bit precision in the analog-to-digital converter output signal.
Integrating analog-to-digital converters can meet this requirement since they produce very accurate conversions. This occurs because of the averaging of the input signal over selected times through integration and, preferably, the integration would be done continuously to effectively eliminate the repeated sampling aspect of the conversion process and therefore eliminate any constraint on the lowest frequency signal which can be converted.
On the other hand, integrating analog-to-digital converters in many forms thereof are deficient in responding to rapidly changing inputs because of the very integration process which is used to provide accuracy. Performing the integration, or the averaging process, over time results in a delay in providing correct digital signal outputs due to the time taken for convergence of the conversion process, and yet the longer the time taken for those operations the more accurate the conversion performed. Thus, there is desired an integrating analog-to-digital converter which can provide an exact conversion periodically at a fast rate during the conversion process but which retains the integration process for the input analog signal to result in an accurate conversion.